Silver halide grains which are tabular in habit are known in the art. Such grains are thin, flat, twinned octahedra of triangular or hexagonal shape. Some tabular grains are found in many conventional emulsions, but only a very small proportion of the grains are tabular in such emulsions. Single jet emulsions having relatively wide grain size distribution generally contain some tabular grains. It has been desirable to remove such tabular grains or avoid their formation to narrow the grain size distribution of such emulsions to yield so-called monodisperse emulsions since it is known that tabular grains grow at substantially faster rates than grains of other crystal morphologies.
The use of an emulsion having a preponderance of tabular grains in a photographic element is, however, considered desirable. For example, the thin, flat crystal morphology provides silver halide grains having substantially increased surface to volume ratios compared to equivalent volume conventional "chunky" silver halide grains. It is believed that these enhanced ratios can permit higher levels of sensitizing dye to be employed (per given mass of silver halide) resulting in an improved photoreceptor and thus increased photographic speed. This described increased surface area to volume ratio also provides for an increased potential for reactions with developers, antifoggants, chemical sensitizers and other photographic addenda, as well as enhanced covering power.
Photographic emulsions having tabular grains are reported in the literature, but generally suffer from various deficiencies in method of preparations or final product. Such deficiencies include preparation of grains that are too large for use in a photographic element (not photographically useful) or grains that are of too low an aspect ratio that they cannot take advantage of, for example, increased sensitizing dye levels. Many of such preparations reported in the literature are cumbersome or difficult to control.
In C. R. Berry and D. C. Skillman, "Fundamental Mechanisms in Silver Halide Precipitation", The Journal of Photographic Science, vol. 16, (1968) pgs. 137-147, a method is described wherein tabular grains are prepared by a double jet precipitation where 2N solutions of silver nitrate and potassium bromide are added simultaneously to a 3% gelatin solution at 30.degree. C. and pAg of 8. Stable cubes of silver bromide are formed. At the end of the precipitation, the formation and growth of the platelet grains occurs by raising the temperature to 50.degree. C. and the pAg to 9.9 by adding excess potassium bromide to the system. Addition of the excess bromide yields twinning and after a prolonged Ostwald ripening period tabular grains are produced. Referring to the photomicrograph on page 143 it will be noted that while there are present a number of large grains described by the authors as "thin tabular crystals" there are also visible a large number of very small grains which were formed by the double jet precipitation. Thus, it would appear that if the Ostwald ripening was halted when the tabular grains are at a size where they are photographically useful there would be present an undesireably large population of the very small grains. Conversely, carrying out the Ostwald ripening to substantially eliminate the very small grains, extremely large tabular grains would result, beyond the range of photographic usefulness.
In C. R. Berry, S. J. Marino and C. F. Oster, "Effects of Environment on the Growth of Silver Bromide Microcrystals", Photographic Science and Engineering, vol. 5, no. 6 (Nov.-Dec. 1961) pages 332-336, methods for the growth of tabular silver bromide grains are described. The method involved the simultaneous addition at equal rates of silver nitrate and potassium bromide solutions to a mixing vessel containing a gelatin solution. A variety of growth modifiers were added to the gelatin solution. It is stated that the only condition that produced an appreciable change in grain size and shape was the addition of potassium bromide to the mixing vessel to produce a pAg of 10.3 during precipitation. Approximately 20% of the crystals formed were tabular, with the remaining being octahedra and needle-like. The article also states that at a pAg below 10.3, i.e. at 9.7, a mixture of small cubes, tetrahedra and octahedra are formed, but no tabular grains. All tabular formation and growth takes place during precipitation. In addition, it is also pointed out that only untwinned octahedra are formed when ammonia is present as a growth modifier.
U.S. Pat. No. 4,150,994, issued Apr. 24, 1979, is directed to a method of preparing a silver halide emulsion wherein the crystals are of the twinned octahedral or cubic type which comprises forming a dispersion of silver iodide nuclei in gelatin, mixing silver nitrate and bromides or chlorides to form twinned crystals, Ostwald ripening in the presence of silver halide solvent solution and controlled silver ion concentration, optionally causing the twinned crystals to increase in size by adding further silver salt solution and halide salts and then removing the water-soluble salts formed and chemically sensitizing the emulsion.
U.S. Pat. No. 4,063,951, issued Dec. 20, 1977, is directed to silver halide emulsions wherein the crystals are of tabular habit bounded by (100) cubic faces and which have an aspect ratio of from 1.5:1 to 7:1. The crystals are prepared by adding an aqueous solution of a water-soluble halide and an aqueous solution of silver nitrate to an aqueous colloid medium by a double jetting technique at a controlled pAg value of between 4.0 and 8.0 and at a controlled constant temperature within the range of 35.degree.-75.degree. to produce monodispersed untwinned seed crystals and then allowing the seed crystals to increase in size by Ostwald ripening in the presence of sufficient ammonia and alkali halide to favour the cubic habit of the crystals at a temperature of between 35.degree. to 70 .degree..
In F. H. Claes "Crystal Growth and Reactivity Sites of Twinned-Tabular Grains", F. H. Claes, Photogaphische Korrespondenz Band, 101, 139-144, (1965), it is stated that the preparation of tabular crystals must occur in the absence of ammonia; if not, a three-dimensional crystal growth occurs. If precipitation does occur in an ammoniacal medium, tabular crystal growth can subsequently be obtained if the ammonia is removed from the emulsion before ripening.
A novel method for preparing tabular grains has now been found which is not susceptible to the deficiencies of the prior art.